Use a better default MAX_GIT_DIFF_LINE_CHARACTERS (#1845)

Tests indicate that line length alone does not make browsers slow, so
increase the default threshold after which diffs get surpressed for line
length from 500 to a more reasonable 5000 characters.

Fixes: https://github.com/go-gitea/gitea/issues/1826

conf/app.ini

@@ -460,7 +460,7 @@ DISABLE_DIFF_HIGHLIGHT = false
 ; Max number of lines allowed of a single file in diff view
 MAX_GIT_DIFF_LINES = 1000
 ; Max number of characters of a line allowed in diff view
-MAX_GIT_DIFF_LINE_CHARACTERS = 500
+MAX_GIT_DIFF_LINE_CHARACTERS = 5000
 ; Max number of files shown in diff view
 MAX_GIT_DIFF_FILES = 100
 ; Arguments for command 'git gc', e.g. "--aggressive --auto"